Atherosclerosis and plaque rupture leading to myocardial infarction remain the leading cause of death worldwide [1]. Inflammation and underlying cellular and molecular mechanisms [2-4] contribute to atherogenesis from initiation through progression, plaque rupture and ultimately, thrombosis. The vulnerable plaque, recently defined by Virmani [5] as “thin-cap fibroatheroma”, results from inflammation and is characterized as having a thin fibrous cap typically less than 65 μm thick, increased infiltration of macrophages with decreased smooth muscle cells, and an increased lipid core size compared to stable plaques [6-8].
Several cellular and molecular events that lead to rupture of thin-cap fibroatheromas are now understood and being utilized to develop novel imaging approaches. Accumulations of macrophages in thin-cap fibroatheromas over-express matrix metalloproteinases (MMPs) [9-12] which are believed to contribute to vulnerability of thin-cap fibroatheromas and increased thrombogenicity [13-15]. Macrophages are an important early cellular marker that indicates the risk of plaque rupture in the coronary, cerebral, and peripheral circulations. Since plaque vulnerability is related to cellular composition as well as anatomical structure, developing a diagnostic method that can simultaneously reveal both composition and structure is desirable to identify vulnerable plaques and would allow in vivo monitoring of macrophage density in longitudinal studies in response to cardiovascular interventions.
Intravascular OCT (IVOCT) is a recently developed catheter-based method for high-resolution intravascular imaging. Of the cardiovascular imaging modalities, IVOCT is the only approach that provides sufficient spatial resolution to image thin-cap fibroatheromas.
However, risk of plaque rupture cannot be easily assessed by only IVOCT images. Two-photon luminescence (TPL) microscopy uses nonlinear optical properties of tissue and has been utilized to image plaque components such as endothelial cells, smooth muscle cells [16], elastin fibers [17,18], oxidized LDL [19] and lipid droplets [20] based on their endogenous autofluorescence. More recently, it has been reported that macrophages loaded with nanoparticles can be detected by TPL microscopy [21,22]. Fiber-based OCT [23,24] and TPL microscopy [25-28] has been reported respectively using photonic crystal fibers to transmit broadband light for achieving higher spatial resolution or to transmit ultrashort pulses for system size minimization. However, a combined fiber-based OCT-TPL system has not been previously realized.